A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The imaging of the pattern including small structures, possibly protected by a pellicle, is very sensitive to dust and other contamination of the patterning device and substrate. Therefore, before imaging, the patterning device (and/or the pellicle protecting the small structures thereof) and substrate are tested for contamination, in particular for particles. In conventional lithographic apparatus, a particle detection system directs a beam of radiation, in particular monochrome radiation, i.e. radiation having substantially one wavelength, on a surface of an object, for example, but not limited to, the patterning device or the substrate. The object and/or the beam move in order to scan the surface of the object. When the beam of radiation engages the surface of the object, the radiation is partially reflected according to physical laws of reflection (an exit angle is identical to an angle of incidence with respect to a fictitious line perpendicular to the surface (the normal)). Another part of the incident radiation may enter the object, such as the patterning device or substrate, and is refracted. In both cases, the beam is anisotropically redirected. When the beam of radiation engages a contaminating particle, the radiation is scattered, i.e. reflected isotropically.
A radiation detector is positioned with respect to the surface and the beam of radiation such that radiation reflected on the surface is not incident on the detector, but a part of the radiation scattered, i.e. being reflected in substantially every direction, by a particle or other contamination is incident on the detector. Thus, the detector receives radiation only when the beam of radiation is scattered by a particle or other contamination.
A part of the radiation incident on the surface of the object enters the object and is refracted, as above mentioned. Inside the object, the beam may be refracted and/or diffracted by the chrome pattern and/or reflected one or more times. Depending on a number of parameters, such as the material, the size, the geometry, and the like, a part of the radiation that entered the object will leave the object again in the direction of the detector. In that case, the detector detects radiation not being scattered by a particle. As a result a detection circuit receiving a signal from the detector determines that a particle is present, although no particle is actually present. Such a detected, but not actually present particle will hereinafter be referred to as a ghost particle.
In other conventional systems for detecting particles, a microscope may be used. Such systems use a microscope to scan the surface and may perform a detailed analysis of any detected particle. However, such systems are expensive and less suitable for mere in-line detection of particles.